[Elecraft] dynamic range questions

[Corboy-Poteet]

Geoff, thank you very much for the response. I am posting to the list
as others may find your comments helpful.

Mike  W5FTD


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Mike,

A couple of comments before I crash - it is past 3 am here!!

> IMD for the K3 (Orion, FTxxx, etc) refers to spurious signals produced
> by operation of a stage (or stages) in the nonlinear portion of its
> transfer curve (or whatever it's called)?

Yes, and the test result should specify which spurious products e.g.
3rd Order.

> The measurement "ground rules" imply two signals: one at fc and the
> other at fc+x, where "x" is 20khz, 10khz, ... ?

This is where the "numbers game" starts! A proper test has both
signals in the passband along with the products, but the great
majority of commercial amateur receivers would not do too well. So
with great reluctance on the part of ARRL the spacing "x" has been
gradually decreased with both signals outside of the passband so that
the product being measured is within the passband.

> The parameter usually being determined is referred to as the two tone
> dynamic range and indicates the point at which third order signals
> resulting from 2fc-(fc+x) and 2(fc+x)-fc begin to appear in the signal
> path?

This would be the 3rd Order dynamic range. The dynamic range is
sometimes measured from the noise floor but not always. This is
because some amplifiers / mixers do not "behave" properly because the
level of the 3rd Order products over some part of the allowed input
level range does not increase at the rate of 3db for every 1db
increase in the level of the input signals.

> fc and fc+x are chosen to put (at least one of) the third order
> signals in the IF passband?

Yes.

> I assume that receiver circuit characteristics before the roofing
> filters (or IF bandpass filter) essentially determine the dynamic
> range. However, dynamic range figures "always" seem to degrade for
> small separations of fc and fc+x. Is this because fc and/or fc+x are
> falling within the IF passband and are producing spurious signals in
> the stages following the roofing filters?

Yes, and this is something that many designers seem to forget.

> For the case of strong signals in the IF passband, are the dynamic
> range tests run with the AGC disabled? In actual operation, wouldn't
> the AGC reduce signal levels below the point where distortion products
> were being generated (in the stages following the roofing filters)?

I believe that most if not all 3rd Order dynamic range tests on
amateur receivers are run with the AGC disabled, but don't quote me!
In actual operation the IMD generated by some types of AGC controlled
amplifier can get worse as AGC is applied. It is up to the designer to
do his job properly!

> The two tone test is only a proxy for the "real world". In actual
> operation, if any signal (or signals) in the RF passband or mixer
> passband exceeds the receiver dynamic range, will ALL of the signals
> in the passband begin contributing third order products?

Yes, how much will depend on individual levels and of course the
receiver design.

> Is the magnitude of the third order distortion products a function of
> the "degree" of the nonlinearity? Can different receiver models with
> the same dynamic range numbers (operating under the same conditions)
> differ considerably in the "signal strength" of junk signals?

Yes is the short answer. This is one good reason why 3rd Order dynamic
range tests should be done with both signals in the passband which can
expose problems which might exist after the IF filter in model A but
not in model B. IMD generated by IF filters can be a problem which
might not be fully exposed if the test signals are widely spaced. Two
major issues are of concern here (1) the effect of the filter on the
dynamic range of the driving stage and (2) the generation of IMD
products within the filter which are passed on to the following IF
cascade.


> When test results are "phase noise limited", what is going on? Is the
> phase noise acting simply as RF noise that masks the spurious signals
> being observed? Or is it acting as a "third signal" that mixes with fc
> and fc+x and causes the production of third order products at a lower
> signal level than fc and fc+x would alone?

Phase noise from a noisy LO has the effect of lifting the noise floor
and is involved in reciprocal mixing. "Phase Noise Limited" means that
the useful performance of the receiver has been compromised by LO
noise, for example the 3rd Order dynamic range is reduced or limited
by phase noise because there is less "headroom" between the increased
noise floor and the point where the receiver runs out of steam.
Reciprocal mixing takes place when the LO phase noise mixes with an
unwanted signal to produce a "noise" IF signal which could mask weak
real signals - an unwanted LO if you like.

I hope that this helps a little.

73,
Geoff
GM4ESD